Al2O3 catalysts

Applied Catalysis B: Environmental 26 (2000) 241–255

Pd–Ce interactions and adsorption properties of palladium: CO and NO TPD studies over Pd–Ce/Al2 O3 catalysts Dragos Ciuparu, Abdelhamid Bensalem, Lisa Pfefferle∗ Chemical Engineering Department, Yale University, New Haven, CT 06520, USA Received 22 February 1999; received in revised form 26 January 2000; accepted 8 February 2000

Abstract We have examined the adsorption of CO and NO on powder Pd/Al2 O3 , Pd–Ce/Al2 O3 and CeO2 /Al2 O3 catalysts, using temperature-programmed desorption (TPD). For CO adsorption on oxidized and pre-reduced Pd–Ce/Al2 O3 TPD profiles are identical to those observed for Pd/Al2 O3 , suggesting that interactions between ceria and Pd have a negligible effect on the adsorption properties of CO. It does, however, affect the oxidation state of the palladium particles. For NO, there are differences between Pd/Al2 O3 and Pd–Ce/Al2 O3 . On oxidized catalysts, Pd/Al2 O3 is more efficient for NO dissociation. However, pre-reduction increases the amount of NO that can adsorb on Pd–Ce/Al2 O3 and react to N2 O and N2 . In comparison with Pd/Al2 O3 , reduced Pd–Ce/Al2 O3 catalysts dissociate NO at relatively high temperatures but they are more reactive and favor N2 over N2 O. © 2000 Elsevier Science B.V. All rights reserved. Keywords: Palladium; Ceria; Pd–Ce interaction; CO; NO; TPD

1. Introduction In the last few years, the use of Pd-based catalysts for three-way catalysis has increased considerably [1]. There are two major reasons for this renewed interest in palladium. First, Pd is by far the cheapest noble metal in the market. Second, to achieve faster light-off performance, particularly for HC conversion, Pd-based catalysts offer significant advantages over the traditional three-way catalysts [2–6]. The use of Pd is considered attractive because of its ability to catalyze the oxidation of HC at temperatures significantly below that of comparable Pt–Rh catalysts, and because of its high temperature durability [7]. How∗ Corresponding author. Tel.: +1-203-432-4377; fax: +1-203-432-7232. E-mail address: [email protected] (L. Pfefferle)

ever, it is well known that Pd-only catalysts show insufficient NOx conversion at stoichiometric conditions [3,4,8]. Studies have shown that with net oxidizing conditions, Pd favors the CO–O2 reaction over the CO–NO reaction as opposed to Rh. As a consequence when using Pd the air/fuel window is narrowed due to the poor NOx conversion on the lean side [9]. Efforts have been focused on improving the catalytic performance of Pd. Changes in the washcoat crystallite structure by use of perovskites were found to improve the activity of Pd [10]. The classical promotion by ceria has been extended to lanthanum oxide and other rare earth and alkaline metal oxides [8,11–13]. Supports other than Al2 O3 such as yttria-stabilised zirconia [14] and ceria–zirconia [15] have also been used. Finally, a structure of double layered Pd catalysts was also developed and was found to improve Pd activity [5]. However, only a few efforts have been devoted

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to understand the effect of the interactions between Pd and ceria in Pd–Ce/Al2 O3 [16] and in Pd/CeO2 system [17–19]. Recently, Tagliaferri et al. [20] showed that promotion of Pd/Al2 O3 by CeO2 is most effective under cycling operation. However, no conclusion was made concerning the oxidation state of Pd because the situation is more complex as a consequence of possible formation of PdO. For example, the role of PdO formation in methane oxidation is still a matter of debate [21]. It has been suggested that synergism between noble metals and surface species of ceria would lead to a better catalytic performance in exhaust gas treatment [22,23]. Hu et al. [5] showed recently that ceria retards the reduction of Pd oxide through Pd–Ce interactions. They found this effect to be responsible for the low activity of Pd in NOx conversion while the influence on CO conversion was negligible. The effect of ceria on the adsorption properties of Pd was investigated recently by Cordatos et al. [19] by temperature-programmed desorption (TPD) of CO and NO on model Pd/ceria catalysts. They found that Pd–Ce interactions influence the adsorption of NO but have a negligible effect on CO adsorption. The goal of this work is to study the Pd–Ce/␥-Al2 O3 system in order to understand the effect of ceria and preparation method on the redox and adsorption properties of Pd. To this aim, Pd/Al2 O3 , Pd–Ce/Al2 O3 and CeO2 /Al2 O3 catalysts were investigated by TPD of NO and CO. NO TPD was used to investigate the effect of ceria on the activity of Pd for NO dissociation, and on the formation of N2 O, N2 and NO2 . In addition to the effect of ceria on CO adsorption, CO TPD was also used to probe the oxidation state of Pd and,

indirectly, to obtain information on the effect ceria on the thermal decomposition of PdO.

2. Experimental 2.1. Catalysts preparation and characterization The following starting materials: Pd(NO3 )2 ·H2 O, Pd(acac)2 , Ce(NO3 )3 ·6H2 O and Ce(acac)3 ·H2 O, all of high purity, were obtained from ALDRICH. ␥-Al2 O3 from ALFA was used as support which after calcination at 800◦ C for 12 h showed a BET area of 105 m2 /g. The catalysts were prepared by different methods (incipient wetness impregnation, grafting in organic solution) and loadings. All the catalysts were dried at 120◦ C for 15 h and then calcined in air at 550◦ C for 15 h. CeO2 /Al2 O3 and Pd/Al2 O3 were prepared by impregnation using aqueous solutions of Ce(NO3 )3 ·6H2 O and Pd(NO3 )2 ·H2 O. The first Pd–Ce/Al2 O3 catalyst was prepared by coimpregnation using nitrate precursor of Pd and Ce on the Al2 O3 support (PdCeAl-CI). The second catalyst was prepared by successive impregnation. After calcination, the CeO2 /Al2 O3 solid was reimpregnated with aqueous solution of Pd nitrate, dried and again calcined (PdCeAl-SI). Finally, one catalyst was prepared by co-grafting of palladium acetylacetonate and Cerium acetylacetonate from benzene solution under reflux conditions (PdCeAl-CG). Details about this procedure of grafting were given elsewhere [17,18]. The compositions and the properties of the prepared catalysts are listed in Table 1.

Table 1 Prepared catalysts: preparation method, composition and their properties Code

Catalyst

Pd (wt.%)

Ce (wt.%)

Preparation method

BET area (m2 /g)

d1 (nm)a

d2 (nm)b

CeAl-I PdAl-I PdCeAl-CI PdCeAl-SI PdCeAl-CGc

CeO2 /Al2 O3 Pd/Al2 O3 Pd–Ce/Al2 O3 Pd–Ce/Al2 O3 Pd–Ce/Al2 O3

– 1.3 0.76 0.88 0.14

10 – 7.0 9.2 6.5

Impregnation Impregnation Co-impregnation Successive impregnation Co-grafting

97 103 94 93 96

– – 20–30 20–30 ndd

– 4.5 3.5 3 2.5

a

Only the big particles were detected by TEM. Average particle size calculated by the DRS technique. c Only catalyst where ceria alone was detected. d nd: not detected, Pd loading below TEM detection limit. b

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The specific surface areas of the catalysts were measured by nitrogen physisorption method, with a Micromeritics 2100E Accusorb instrument. From the adsorption isotherm at liquid nitrogen temperature (77 K) the surface areas of the catalysts (see Table 1) were calculated using BET equation. Calcined Pd–Ce/Al2 O3 catalysts were examined by transmission electron microscopy (TEM), with a point resolution of 1.9 A. Due to the poor contrast between Pd and Ce, it was very difficult to distinguish between PdO and CeO2 . Particles of approximately 20–50 nm in size were detected on the catalysts prepared by coimpregnation and successive impregnation methods. However, the statistics are rather poor since only few particles are seen in any given region. There was no indication of presence of PdO on the catalyst prepared by cografting, so it appears that the amount of palladium in this catalyst is below detection. EDS analysis indicates for the PdCeAl-CI and PdCeAl-SI catalysts the Pd is present on both Al2 O3 and CeO2 /Al2 O3 phases. Moreover, the Ce signal was detected alone only for the co-grafted catalyst, indicating the presence of pure CeO2 particles. Diffuse reflectance spectroscopy (DRS) was used to measure PdO average particle size. This size characterization is based on the relation between domain size and the bandgap energy (Eg ) of an insulator or semiconductor [24–26]. An analytical function has been developed that models the bandgap behavior of particles in the size range from about 100 down to about 2 nm [26]. This procedure was already applied to PdO (p type semiconductor) by Weber et al. [27], using the hyperbolic band model of Wang et al. [26], a bulk bandgap energy of 0.8 eV and an effective mass of 0.1 me, the latter being the value estimated from the equation given by Harisson [28]. The DRS spectra were acquired on a Hewelett–Packard 5482 A UV–Visible spectrophotometer equipped with a specially designed Praying Mantis diffuse reflection attachment (DRA) supplied by Harrick. We have adopted the procedure developed by Weber [29] to determine the absorption edge. The graphs of [F(R∞ )×hλ]2 versus hλ, where F(R∞ ) is the Kubelka Munk function for an infinitely thick sample and hλ is the energy of the incident photon, are fitted to a sum of an arctangent curve and, when necessary, a Gaussian feature [29]. The obtained average particle sizes are given in Table 1.

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2.2. Temperature-programmed desorption (TPD) experiments The reactor consisted of a horizontal alumina tube, 300 mm long and with 4 mm i.d., encased in a tubular furnace. The catalyst — 350–400 mg — was sealed in the middle of the heated zone with alumina wool. Two chromel-alumel type K thermocouples (Omega), one at the front and one at the back of the catalyst, were used for temperature reading. The gases were introduced into the reactor using mass flow controllers (Brooks 5850). Reactants and products were analyzed on-line by a quadropole mass spectrometer (Hewelett-Packard, MS 5971A model). All TPD measurements were carried out with a heating rate of 100◦ C/min and a He flow rate of 300 cc/min. The gases used were: 10 vol.% CO, 10 vol.% NO, 10 vol.% C3 H6 , 1 vol.% C3 H6 , 1 vol.% O2 , 5 vol.% O2 and 5 vol.% H2 in He, and pure He, O2 and CO, all supplied by Airgas and having less than 1 ppm total impurities. Prior to all experiments the catalysts were calcined in situ in an O2 flow at 550◦ C for 2 h, to ensure that all the carbonaceous species deposited on the surface are oxidized. Subsequently the surface was cleaned by evacuation in He at 550◦ C for 0.5 h and then cooled to room temperature (RT) in He. After that pre-treatment, the catalysts were named ‘oxidized’. The ‘pre-reduced’ samples were obtained by calcination in O2 flow (2 h), evacuation in He flow (0.5 h) and reduction in CO flow (2 h) at 550◦ C, and then cooled to RT in He. The ‘pre-aged’ samples were obtained using the following pre-treatment: calcination (2 h) and evacuation (0.5 h) at 550◦ C, aging in stoichiometric CO/NO/C3 H6 /O2 mixture (2 h) and evacuation in He (0.5 h) at 850◦ C, and then cooling to RT in He. Temperature-programmed desorption (TPD) was performed using CO and NO as the adsorbate gas. Adsorption was carried out at room temperature in 10% CO–He or 10% NO–He flow (25 cc/min) for 0.5 h. The catalyst was then exposed to He for 0.5 h at room temperature to remove all the physically adsorbed species before starting the temperature program. CO was adsorbed on oxidized, pre-reduced and pre-aged samples while NO adsorption was performed only on oxidized and pre-reduced samples. Since it is well known that re-adsorption effects may be difficult to eliminate during TPD experiments

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3.1. Adsorption of CO

CO2 was the product of co-adsorbed oxygen, it would appear in a peak centered at a lower temperature [19]. In the case of the pre-oxidized catalyst, a potential source of CO2 consists in the oxidation of the adsorbed CO with oxygen from the bulk of the incompletely reduced PdO particles. However, as described in previous studies [32,33], metallic palladium particles favor the formation of CO2 also by means of the CO disproportionation reaction: 2 CO(g) →CO2(g) +C(s) . The TPD spectra after CO adsorption on the PdAl-I catalyst are shown in Fig. 1. For all catalysts, CO desorbs in a single peak. The CO peak maximum occurs at approximately 205◦ C for the oxidized sample and 308◦ C for the pre-reduced and the pre-aged samples. The pre-oxidized sample, however, also has a shoulder peak centered near 308◦ C. These peaks are assigned to multiple coordinated CO rather than linear CO because, according to previous IR studies [18,31,34], linear species start to desorb at room temperature. Fig. 1 shows that the oxidized PdAl-I exhibits a peak maximum around 205◦ C, 100◦ C lower than the pre-aged and pre-reduced samples, and only a shoulder centered at the temperature corresponding to the maximum desorption of pre-aged and pre-reduced catalysts. It is known that CO adsorbs strongly on metallic Pd, but incompletely reduces the PdO at RT [31]. This clearly indicates that after pre-aging in the standard CO/NO/C3 H6 /O2 stream at 850◦ C or pre-reduction in

It should first be noted that although the catalysts underwent different treatments (i.e. pre-reduction, pre-oxidation or pre-aging) all the samples expose a metallic surface after 30 min of flowing CO at RT, as reported elsewhere with ceria supported catalysts [18] and demonstrated by Tessier et al. with alumina supported catalysts [31]. They observed by means of UV–Vis DRS spectroscopy that on calcined samples (PdO/Al2 O3 ) the reduction by CO is immediate and limited. Consequently, the pre-oxidized samples are assumed to have an oxide core covered with a metallic layer. Therefore, the differences that appeared among the CO-TPD profiles as a function of pre-treatment history are likely related to differences in the bulk composition of catalyst particles supported on different supports. For all of the catalysts a significant fraction of the adsorbed CO desorbs as CO2 . The catalyst was not exposed to oxygen prior to CO adsorption; furthermore if

Fig. 1. TPD profiles following CO adsorption at room temperature on PdAl-I catalyst. Open symbols for CO desorption and close symbols for CO2 desorption.

in flowing carrier setup [30], we have checked the effect of re-adsorption by carrier flow rate variation. We found that 300 ml/min He flow rate is high enough to avoid a high CO partial pressure in the gas phase during desorption. This effect, however, still contributes to peak broadening. 2.3. CO pulse experiments CO pulse experiments were performed in a fixed bed alumina reactor with 4 mm i.d., placed in an electric heater. The chromatograph carrier (He Grade 5.0 from Airgas) pass through the catalytic bed before going to the chromatographic column. The column effluent is passed through a methanizer and is fed to an FID detector. This experimental setup allows to determine the CO and CO2 concentrations in the reactor outlet stream. The reactant could be injected in pulses or continuously, depending on the goals. For a typical run 350 mg of catalyst were used. The carrier flowrate was 450 ml min−1 and the CO injection loop had 20 ␮l. All the experiments were performed at constant temperature and atmospheric pressure.

3. Results

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CO at 550◦ C, palladium is in the metallic state, while in the case of the pre-oxidized catalyst, the palladium particles are in a different state. The most probable state is a metallic palladium superficial layer on a PdO core. This is consistent with many methane oxidation studies showing that oxidized PdO is reduced in air or mixtures of a few percent hydrocarbon in air at 850◦ C in a time scale of minutes, and with CO adsorption studies reporting the PdO supported on alumina is partially reduced by the CO at RT. Confirmation of the incomplete reduction of PdO particles was obtained from CO pulse experiments. A previously oxidized sample in flowing O2 at 550◦ C for 12 h was purged with He at the same temperature for 30 min, cooled down to RT in flowing He, and subjected to CO pulses at different temperatures, as shown in Fig. 2. The reactor effluent composition was monitored by gas chromatography. At RT the CO2 concentration sharply decreases reaching undetectable levels after the first two pulses. The same sharp decreases in CO2 concentrations were observed both at 100 and 170◦ C, but the initial CO2 concentrations were higher. At 300◦ C the initial CO2 concentration was very high and decreases with a relatively low slope. In contrast, a catalyst sample was reduced in flowing methane at 550◦ C until no CO or CO2 traces were detected in the reactor effluent. This catalyst did not produce CO2 following CO pulses at RT and 100◦ C, and produced only small

Fig. 2. The CO conversion after successive CO pulses at different temperatures over a pre-oxidized alumina supported catalyst.

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Fig. 3. The CO conversion for successive CO pulses at 300◦ C over a pre-reduced Pd/Al2 O3 catalyst.

amounts of CO2 after pulses at 170◦ C. After the first pulse at 300◦ C it gave only CO2 (see Fig. 3). The CO2 concentration observed in the pulse experiments at 300◦ C decreases at a constant rate and stabilizes at about one-tenth of the initial concentration and remains constant for the next 10 pulses, suggesting no further deactivation. Following the CO pulses, the catalysts were heated in He at 550◦ C and held for 30 min in order to evacuate all possible CO or CO2 adsorbed molecules. When subjected to O2 pulses, significant amounts of CO2 were detected in the outlet stream, confirming the presence of surface carbonaceous deposits. These experiments confirmed that the observed CO2 was produced both by CO oxidation by oxygen from the catalyst and by CO disproportionation in the case of the pre-oxidized samples, and by CO disproportionation alone in the case of completely reduced catalyst particles. A second experiment was performed to confirm that the CO2 observed is also the product of CO disproportionation. The pre-oxidized PdCeAl-CI catalyst was used in a CO-TPD experiment from room temperature to 900◦ C at 100 K/min. Immediately following the CO-TPD this catalyst was cooled down to RT and subjected to a second temperature program at 100 K/min in a flow of 5% H2 –He. Only one broad methane desorption feature was observed around 500◦ C. This indicates significant carbon deposition on the surface confirming that CO disproportionation reaction was an important source of CO2 , since the surface oxygen

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was previously consumed by CO oxidation during the TPD experiment preceeding the hydrogen flow. It is worthy to note that the CO and CO2 peak areas observed in Fig. 1, correlate with the pre-treatment history of each sample. The pre-oxidized sample, which underwent the mildest treatment (oxygen at 550◦ C, cooling at RT in He, and 30 min under flowing CO at RT, a mild reducing agent) has the highest CO+CO2 desorption capacity. The desorption capacity of the same amount of pre-aged catalyst is lower. The pre-reduced catalyst desorbs the lowest specific amount of carbon oxides, most likely due to the carbon deposited on its surface during the reduction process at 550◦ C in flowing CO. Meanwhile, the relative ratio between the CO2 and CO peak areas is almost the same for the pre-oxidized and pre-aged samples, and is considerably lower for the pre-reduced catalyst. This is most probably due to the intense poisoning of the sites most active in CO disproportionation. To investigate the effect of CeO2 , TPD experiments were performed on Pd–Ce/Al2 O3 catalysts. The spectra obtained are similar to those obtained for the Pd/Al2 O3 catalyst. For each sample, CO and CO2 had the same peak maximum. As shown in Fig. 4, CO TPD curves of Pd–Ce/Al2 O3 catalysts are characterized by a very broad peak, with a maximum near 200◦ C for the oxidized (a) and pre-aged (c) catalysts and at about 300◦ C for the pre-reduced (b) catalysts. This peak is due to CO strongly adsorbed on Pd sites (e.g. CO multiple coordinated). A contribution from CO adsorbed on CeO2 cannot be ruled out since the peaks are very broad. Although the characteristic desorption temperatures are the same as for alumina supported catalysts, the pre-aged ceria supported catalysts exhibit desorption spectra similar to the pre-oxidized catalysts. Thus, in contrast to the pre-aged Pd/Al2 O3 catalyst, palladium in the Pd–Ce/Al2 O3 catalytic system is observed to be in an oxidized state after the pre-aging treatment in CO/NO/C3 H6 /O2 at 850o C as confirmed by UV–Vis DR spectroscopy. These observations suggest that the presence of ceria stabilize the palladium oxide phase. This was also confirmed on different model catalysts using UV–Visible diffuse reflectance spectroscopy. As we found in the case of alumina supported catalysts, the desorption peaks areas correlate with the treatment history of the samples: the pre-oxidized sample has a systematically higher CO desorption peak

Fig. 4. CO-TPD profiles following CO adsorption at room temperature on PdCeAl-CI, PdCeAl-CG and PdCeAl-SI catalysts: (a) pre-oxidized, (b) pre-reduced and (c) pre-aged.

area than the pre-aged, while the pre-reduced catalyst has the lowest desorption capacity, independent of the preparation method used. Significant differences were observed when CO TPD experiments were performed on the CeO2 /Al2 O3 system (Fig. 5). The oxidized sample (a) shows a broad CO desorption peak centered around 335◦ C. The pre-reduced sample (b), however, shows two well resolved peaks centered at 160 and 400◦ C. Two peaks were observed using FTIR spectroscopy for ceria supported catalysts and were attributed to linear and bridged CO molecules adsorbed on Ce3+ ions [35]. The absence of the low temperature CO peak around 160◦ C on the reduced PdCeAl-CI and PdCeAl-SI samples suggests that the Ce sites available for CO adsorption on ceria were completely blocked by Pd, or are present at a very low concentration. A weak peak,

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Fig. 5. CO-TPD profiles following CO adsorption at room temperature on the CeAl-I catalyst: (a) oxidized and (b) pre-reduced.

however, was observed at this temperature on reduced PdCeAl-CG sample, which indicates the presence of exposed Ce sites on the initial co-grafted catalyst. This was confirmed by TEM and EDS analysis which indicate that only the co-grafted catalyst present bulk CeO2 particles not covered by palladium. Finally, we should note that since the desorption peaks observed on Pd–Ce/Al2 O3 samples are very broad (see Fig. 5), a contribution from the high temperature peak around 400◦ C observed over CeO2 /Al2 O3 system cannot be totally excluded. 3.2. Adsorption of NO The first observation is that for all the investigated systems NO, N2 , N2 O and NO2 were the four species detected desorbing from the surface as the temperature of the catalyst was increased, independent of the sample pre-treatment history. The desorption spectra obtained for the oxidized and pre-reduced catalysts following NO adsorption are presented in Figs. 6–8. In general, while all of the oxidized ceria containing systems show similar desorption features the Pd/Al2 O3 system behaves differently. Independent of the preparation method, the oxidized ceria–alumina supported palladium catalyst produce NO2 at the same temperature (∼550◦ C) as the alumina-supported catalyst. The ceria–alumina support without palladium produces significantly less NO2 . Among the ceria–alumina supported palladium catalysts, the

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pre-reduced one prepared by successive impregnation shows a high N2 selectivity in a narrow temperature domain, which makes it very interesting for DeNOx applications. Over oxidized PdAl-I (Fig. 6), three NO desorption features were observed in the 200–600◦ C temperature range. N2 and N2 O desorption profiles have one main feature which corresponds to the first desorption feature of NO. Most of the N2 O desorbs from a peak centered at 240◦ C, simultaneously with N2 . It is likely that a significant fraction of the N2 is a fragmentation product of N2 O, a result which has already been reported for Pd/Al2 O3 [36]. The performance of the alumina-supported palladium catalyst was not significantly affected by the pre-treatment history. The desorption spectra look very similar with one exception: the NO2 desorption spectra of the pre-reduced sample shows a lower temperature feature centered at 345◦ C, which is associated with the broad desorption peak in the 250–450◦ C region. In this temperature region the oxidized sample has an unresolved desorption, much less intense than the desorption peak observed at about 585◦ C. The N2 and N2 O desorption features for both oxidized and pre-reduced systems are centered at the same temperature (∼240◦ C) and coincide with the first NO desorption maximum. The high temperature NO desorption peak is centered at the same temperature for both catalysts, with the most intense NO2 desorption peak centered at 585◦ C. The desorption peaks are generally more intense for the oxidized sample, with one exception: the low and high temperature NO desorption peaks have almost equal intensities, but the broad desorption feature between 250 and 450◦ C is more intense for the pre-reduced sample than the better resolved NO desorption peak centered at 400◦ C for the oxidized alumina supported catalyst. As in the case of the PdAl-I catalyst, the oxidized ceria containing systems desorb NO in three well-resolved peaks centered at approximately 180, 415 and 550◦ C, respectively (Fig. 7). It should be mentioned that the ceria–alumina support has slightly higher temperatures for the second and the third desorption peaks. In contrast, the N2 , NO2 and N2 O desorption spectra of the oxidized ceria containing systems are strongly influenced by the presence of palladium. The ceria–alumina support desorbs these

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Fig. 6. TPD profiles following NO adsorption on the PdAl-I catalyst. (The numbers in the upper left corner of each chart give the peak amplification factor).

species at temperatures considerably lower than the supported palladium catalysts. The support desorbs all the three species at about 320◦ C, while palladium supported particles desorbs N2 and N2 O at approximately 435◦ C and N2 O at about 550◦ C. It should be mentioned that the amount of NO2 produced by the support itself is roughly 100 times lower than that produced by the catalysts (see the amplification factor of 100 for the CeAl-I support in Fig. 7). In the NO2 desorption spectra of all the three palladium catalysts a low temperature desorption shoulder could be observed and is attributed to the support. It should also be mentioned that, unlike the PdAl-I system, the N2 and N2 O desorption features are asso-

ciated with the second NO desorption maximum. In the case of PdAl-I catalyst they correspond to the first peak of the NO desorption spectra. This suggests that the oxidized PdAl-I system dissociates NO at lower temperatures than the oxidized ceria containing catalysts. Surprisingly, the ceria–alumina support seems to be more selective for N2 than the palladium catalysts. It desorbs the lowest amount of NO and the highest amount of N2 among the ceria containing systems. This feature, coupled with the very low selectivity for N2 O, makes it interesting for low temperature DeNOx applications. The decrease in N2 selectivity at low temperatures after palladium deposition suggests that

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Fig. 7. TPD profiles following NO adsorption on oxidized ceria containing systems. (The numbers in the upper left corner of each chart give the peak amplification factor.)

the NO dissociation sites on ceria–alumina support are affected by palladium particles. Among the oxidized ceria–alumina supported catalysts the only notable difference is the lower N2 O selectivity observed for the PdCeAl-SI catalyst. The presence of ceria in the reduced catalysts (Fig. 8) makes them to behave very differently from the reduced PdAl-I system. Moreover, the preparation method was observed to strongly influence the behavior of the catalysts in NO adsorption and dissociation. The most characteristic TPD feature of the reduced CeAl-I is the presence of a small N2 O peak at 160◦ C. This feature is related to NO adsorbed on Ce3+ sites. The NO desorption profile of the reduced

ceria–alumina support looks similar to the one observed for the pre-reduced PdAl-I: a broad and poor resolved desorption feature between 100 and 500◦ C and a better resolved desorption peak centered at about 550◦ C. In contrast, the pre-reduced ceria–alumina supported palladium catalysts prepared by different methods show different NO desorption features. The catalysts prepared by impregnation (PdCeAl-SI and PdCeAl-CI) show two low temperature desorption features between 100 and 400◦ C. The maximum desorption temperatures determined by deconvolution for the two systems were 170 and 280◦ C, respectively. The system prepared by co-impregnation, however,

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Fig. 8. TPD profiles following NO adsorption on reduced ceria containing systems. (The numbers in the upper left corner of each chart give the peak amplification factor.)

has also a higher temperature NO desorption peak centered at about 500◦ C, while the PdCeAl-SI does not desorb NO above 400◦ C. The catalyst prepared by co-grafting shows both the low temperature desorption features at 170 and 280◦ C and the high temperature peak at 500◦ C. In addition, it has an intermediary temperature desorption feature, whose maximum was found by deconvolution at 400◦ C. There are no other species associated with the low temperature NO desorption features below 350◦ C. In contrast, N2 , N2 O an NO2 were found to desorb in association with the desorption feature centered at 400◦ C. Among the ceria containing systems, the ceria–alumina support has the lowest N2 selectivity, while the most selective for N2 is PdCeAl-SI. The

PdCeAl-CI is, however, more selective for N2 than the catalyst prepared by co-grafting. Meanwhile, the ceria–alumina support itself is the most NO2 selective at 400◦ C, and the PdCeAl-SI has the lowest NO2 selectivity at this temperature. The other two catalysts show almost equal selectivities for NO2 at 400◦ C. However, it should be noted that, for the pre-reduced ceria containing systems, the NO2 peaks are as much as 400 times less intense than those observed for NO. The NO desorption peak centered at 500◦ C is also associated with N2 , N2 O and NO2 desorption features for the pre-reduced ceria–alumina supported palladium catalysts. Since all the N2 , N2 O and NO2 desorption peaks at 400◦ C observed for the catalysts support are broad, a desorption feature centered at

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500◦ C for this system cannot be ruled out. If exists, it is, however, much less important than those observed for PdCeAl-SI and PdCeAl-CG having about the same intensity with the one observed for the PdCeAl-CI. The NO2 desorption features at 400◦ C are less intense for the palladium containing catalysts than for the ceria–alumina support, suggesting that palladium impedes its formation at low temperatures. It, however, desorbs in larger amounts at higher temperatures from PdCeAl-CG and PdCeAl-SI catalysts. The almost equal intensities observed for the NO2 desorption peaks at 500◦ C indicate that the palladium content does not influence the NO2 formation. It is also worthy to note that, despite the very similar behaviors at temperatures below 650◦ C of these two catalysts, the PdCeAl-SI is the only one system that has a high temperature NO2 desorption peak centered at 750◦ C. PdCeAl-SI also behaves differently with respect to N2 and N2 O desorption. It shows two N2 and N2 O desorption peaks at 400 and 500◦ C, respectively, as the other ceria–alumina supported palladium catalysts, but the N2 selectivities are considerably higher for PdCeAl-SI than for the others. Moreover, the N2 selectivity seems to remain constant in this temperature range, while for the PdCeAl-CG decreases. It should also be noted that the PdCeAl-SI catalyst is the only one that desorbs N2 above 700◦ C, as in the case of NO2 . A slightly different behavior has the PdCeAl-CI whose N2 and N2 O peaks at 400◦ C are more intense than the one centered at 500◦ C. This is in agreement with the absence of the NO desorption peak at 400◦ C and with the low intensity of the NO desorption peak at 500◦ C observed for this catalyst.

4. Discussion 4.1. CO adsorption The presence of two different desorption temperatures observed for the catalysts having the same surface state but different bulk compositions suggests that the observed CO2 results both from CO oxidation with oxygen from particles bulk and CO disproportionation. This is consistent with the TPD profile observed for the pre-oxidized catalyst in Fig. 1, which clearly shows the presence of two desorption features both for CO and CO2 . It is also in agreement with the

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behavior of the pre-oxidized and pre-reduced samples in the CO pulse experiments showed in Figs. 2 and 3. To explain this we propose that, as described by Tessier [31], only the surface layers of relatively large alumina supported PdO particles were reduced by the flowing CO at RT, while the small particles were completely reduced. Therefore, after CO adsorption at RT on the pre-oxidized alumina supported catalyst there are two different populations of particles: small completely reduced palladium particles and larger PdO particles covered by reduced metallic layers. When temperature increases, the mobility of the bulk oxygen increases, reaches the surface and oxidizes the adsorbed CO molecules. Since the oxidation proceeds from the particle surface towards the external layers of adsorbed CO, the superior layers of adsorbed carbon monoxide are desorbed together with the resulted CO2 . Overheating of the catalyst particles caused by oxidation reaction would also result in a more rapid CO desorption. In return, the small, completely reduced particles will retain the adsorbed CO until higher temperatures are reached, at which CO molecules both desorb and disproportionate to give CO2 and solid carbon particles. The small, reduced particles are, therefore, responsible for the shoulder better resolved in the CO2 desorption spectra of the pre-oxidized alumina supported catalyst in Fig. 1, and also visible in the CO desorption spectra of the same catalyst. The present TPD results show that CO adsorption on Pd–Ce/Al2 O3 catalysts is not structure sensitive since we have obtained qualitatively similar results on three Pd–Ce/Al2 O3 catalysts prepared by different methods, when subjected to the same pre-treatment. Over oxidized and pre-reduced catalysts, the TPD spectra for the Pd/Al2 O3 system are similar to those obtained for the Pd–Ce/Al2 O3 system. This suggests that there is no evidence of a Pd–Ce interaction affecting CO adsorption. Cordatos and Gorte [19] have concluded that electronic interactions between Pd and ceria have negligible effect on CO adsorption over model Pd/ceria catalysts. We have previously studied Pd–Ce interactions, using FTIR spectroscopy, and demonstrated that electron transfer from ceria to highly dispersed Pd particles considerably affects CO adsorption over powder Pd/CeO2 catalysts [18]. We have also shown elsewhere [18] using UV–Visible diffuse reflectance spectroscopy that interactions between Pd and bulk ceria favor the reduction of PdO, where as ceria in the

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Pd–Ce/Al2 O3 catalyst in this study is found to stabilize the oxide. It is consistent with the current results that, in a reducing atmosphere, the Pd–Ce interactions susceptible to occur for the Pd–Ce/Al2 O3 system are different from the Pd–Ce interactions (such as an electron transfer from ceria to Pd [18]) observed for Pd/CeO2 system. This explanation is also supported by the results of Hu et al. [5] who found that Pd–Ce interactions in three-way catalysts (alumina present) retards the reduction of Pd oxide. For the same catalyst, the temperature at the CO desorption peak maximum depends on the pre-treatment of the catalyst, which determines the oxidation state of the palladium particles. Over pre-reduced samples, which are believed to be in a fully reduced state, the peak is centered at temperatures 100◦ C higher than over oxidized samples. This result is expected since it is known from previous studies [18,31] that CO strongly adsorbs on metallic palladium and reduces the superficial layers of palladium oxide particles to an extent which depends on the particle size, CO partial pressure, support, time of exposure, etc. Over pre-aged catalysts, CO TPD spectra show notable differences between the Pd/Al2 O3 and Pd–Ce/Al2 O3 systems. Fig. 1 shows that both pre-reduced and pre-aged PdAl-I samples exhibit a peak maximum at 310◦ C. This behavior indicates that palladium is in the metallic state after aging pre-treatment at 850◦ C. On the other hand, the pre-aged Pd–Ce/Al2 O3 exhibits CO desorption peaks at lower temperatures (see Fig. 4), indicating that some portion of the palladium is in an oxidized state. We have confirmed these results on model catalysts using diffuse reflectance spectroscopy. Therefore, we can conclude that the thermal decomposition of PdO is retarded when CeO2 promotes Pd/Al2 O3 . Recently, Farrauto et al. [37] found that PdO supported on ␥-Al2 O3 starts to decompose at 810◦ C while PdO supported on CeO2 starts to decompose at 775◦ C, in air (1 atm). Our results on the Pd/Al2 O3 system confirm those of Farrauto et al. On the other hand, our results on Pd–Ce/Al2 O3 compared to those obtained by Farrauto et al. on Pd/CeO2 again support the idea that the Pd–Ce interactions observed to occur in Pd–Ce/Al2 O3 are different from Pd–Ce interactions in Pd/CeO2 system. The PdO core covered with metallic palladium layer proposed for the oxidized catalysts after CO adsorp-

tion at RT is also consistent with the behavior of ceria containing catalysts. The pre-aged samples, reduced to a partial extent, have a slightly higher temperature for CO desorption maximum than that observed for the pre-oxidized samples. This is most likely because the superficial reduced layer in the pre-aged samples is thicker than that produced by reduction with CO at RT. 4.2. NO adsorption TPD after NO adsorption at room temperature on oxidized and pre-reduced catalysts gave NO, N2 O, N2 and NO2 as the desorption products. Therefore, NO dissociates over Pd/Al2 O3 , Pd–Ce/Al2 O3 and CeO2 /Al2 O3 . Since no oxygen was detected to desorb from the investigated systems, we believe that the oxygen balance is realized either by NO2 formation — in the case of oxidized catalysts, or by both NO2 formation and support oxidation — in the case of reduced catalysts. Unlike PdAl-I, the desorption profiles for ceria containing catalysts present many features over a large temperature range 100–700◦ C. The complexity of these spectra is due to the reactivity of both palladium and ceria for NO decomposition. Regarding the adsorption and reaction properties of Pd–Ce/Al2 O3 , the TPD results show that there are significant changes due to the presence of ceria, mainly after reduction pre-treatment. While one might expect the oxidized sample to yield more NO2 than the reduced catalysts, higher N2 O and N2 selectivities would be expected for the reduced samples. This is the case with ceria containing samples. In contrast, the reduced alumina supported palladium catalysts show lower peak intensities for all desorbing species. This can be explained by means of lower active surface area for reduced samples. Since the catalysts were reduced in flowing CO at 550◦ C, the surface was shown to be covered with carbon resulting from CO disproportionation. Because the amount of available oxygen is much larger for ceria containing catalysts, the CO disproportionation competes with CO oxidation by oxygen from the support, mainly at the palladium–ceria interface where the oxygen concentration is considerably higher than on metallic particle surface. Consequently, the amount of carbon deposited on the catalyst surface is likely

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smaller for ceria containing palladium catalysts and the loss in active surface is less important. This is consistent with the higher N2 and N2 O selectivities observed for the reduced versus oxidized ceria containing catalysts. This behavior suggests that the N2 O formation takes place on sites located at palladium ceria interface, consistent with considerably lower N2 and N2 O selectivities observed at the same temperatures for the reduced alumina supported palladium catalyst and for the co-grafted catalyst with large ceria particles. When performed over an oxidized catalyst, NO desorption indicates the presence of reversibly adsorbed NO which can desorb from the surface in the low-temperature region (100–300◦ C) as well as in the mid (300–450◦ C) and high (450–650◦ C) temperature regions. The three distinct features in the NO desorption profiles suggest that there are at least three types of sites available for NO adsorption. The presence of two types of adsorbed NO species over Pd-catalysts has been reported in the literature earlier [38–40] and these have been attributed to monomeric and dimeric NO adsorbed species. The high temperature feature (dimeric) has been proposed to be responsible for N2 O formation. Three species have also been observed previously [41,42] and have been attributed to linear and bridged NO species [43]. In our TPD experiments performed on oxidized Pd–Ce/Al2 O3 catalysts, a distinct feature due to N2 O was observed which corresponds to the mid-temperature desorption feature of NO. This indicates clearly that, of the three types of NO adsorption sites, only the low temperature site cannot be associated with N2 O formation. Indeed, the first NO feature (NO weakly adsorbed) desorbs before the temperature of NO decomposition is reached. This explanation is based on the assumption that NO starts to decompose at the temperature at which N2 starts to desorb in TPD [36]. Over a Pd/Al2 O3 catalyst however, N2 O starts to desorb at 100◦ C suggesting that the low-temperature NO site can be involved in N2 O formation as well as the two other sites. In addition, this result also indicates that the low-temperature NO peak, observed over all catalysts, cannot be attributed to a single NO specie, the same for all catalysts. Moreover, our results show that, for all oxidized Pd catalysts, NO2 formation is associated with the high temperature NO feature.

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Significant differences were observed when TPD experiments were performed over pre-reduced catalysts. The pre-reduced Pd–Ce/Al2 O3 system was observed to be more reactive than the pre-reduced Pd/Al2 O3 system. In addition, the observed NO desorption profiles depended on the preparation method of the catalyst. The formation of N2 O is shown to be associated with a number of adsorption sites in the mid- and high-temperature regions. It is clear from these results that ceria considerably affects the properties of palladium for NO adsorption. Our TPD results also show that the effect of reducing Pd–Ce/Al2 O3 catalysts is to increase the fraction of NO which dissociates. However, the same reducing pre-treatment decreases the fraction of NO which dissociates over the Pd/Al2 O3 system (Fig. 9). Moreover, reduction favors N2 formation over N2 O on Pd–Ce/Al2 O3 catalysts (Fig. 10). In addition, this figure shows also that increasing Pd loading favors the formation of N2 over N2 O. This interesting effect was recently observed by Tagliaferri et al. [20] on industrial Pd-based TW catalysts. The results reported here for NO suggest that reactions of this molecule will also be strongly affected by the presence of ceria under reducing conditions. Cordatos and Gorte investigated NO adsorption on model Pd/ceria catalysts and concluded to no evidence for electronic interactions [19]. They have suggested that lattice oxygen from CeO2 at the metal–ceria boundary provides oxygen to the metal for the oxidation reaction, while reduced sites at the boundary

Fig. 9. Comparison of the performances of the catalysts for NO dissociation. Influence of reduction and effect of ceria: (a) oxidized and (b) pre-reduced.

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Fig. 10. Comparison of the performances of the catalysts for N2 O formation. Influence of reduction and Pd loading: (a) oxidized and (b) pre-reduced.

provide adsorption sites for NO. In addition to the differences between Pd/CeO2 and Pd–Ce/Al2 O3 this picture is a potential interpretation for our TPD results. In fact, reduction of Pd–Ce/Al2 O3 catalysts induces new NO adsorption sites depending on the preparation method and the distribution of the Pd, Ce and Al2 O3 phases. Those sites may be attributed to Pd sites at the Pd–CeO2 interface. Another potential explanation is that, in an oxidizing atmosphere (e.g. NO), electron transfer from Pd to ceria can also be invoked to explain the high-temperature N2 O desorption peaks due to NO strongly adsorbed on the surface, as explained above. Electron transfer from PdO to ceria was already detected for PdO–CeO2 /Al2 O3 in an oxidizing atmosphere by Le Normand et al. [44]. This interaction is expected to increase the strength of NO adsorption (NO electron donor). However, if such interactions occur one can expect it should be more pronounced on oxidized catalysts (Ce4+ electron acceptors; Ce3+ electron donors), and this is definitely not the case in the present study. On the other hand, electron transfer from ceria to Pd is less probable to occur in oxidizing atmosphere (NO). Such transfer should decrease the strength of adsorption of NO, an effect which is accompanied by reinforcing of the N–O bond, thus decreasing the NO dissociation activity. So, it appears that there is no evidence for electronic interactions (e.g. electron transfer) between Pd and ceria to explain the effect of reduction on the properties of palladium for NO adsorption. It, therefore, seems

reasonable to suggest a similar a picture to that invoked by Cordatos and Gorte for model Pd/ceria catalysts [19]. However, the differences in the desorption features observed for the reduced catalysts prepared by different methods suggest that the active sites are located rather at the Pd–ceria boundary than on palladium and/or ceria particles. Our results indicate that controlling the preparation conditions, high N2 selective catalysts could be obtained. This is the case of the reduced catalysts prepared by successive impregnation, which showed a high selectivity in a narrow temperature domain (between 300 and 500◦ C). Because in both the TEM investigations and UV–Vis characterization the PdCeAl-CI and PdCe-SI looked similar but behaved very differently in the NO-TPD experiments, our results indicate that the NO dissociation site is most likely located at the palladium–ceria interface. Consequently, maximizing the boundary surface is expected to improve the N2 selectivity. For these reasons, further investigations oriented toward the ceria particle size and phases distribution onto the support will provide useful information to optimize the performance of the catalyst.

5. Conclusions 1. CO adsorption and desorption properties for the Pd–Ce/Al2 O3 system are found to be similar to those of the Pd/Al2 O3 system. Pd–Ce interactions were not observed to affect CO adsorption. These interactions do, however, influence the Pd oxidation state. Indeed, our results show, directly and indirectly, that promotion of Pd/Al2 O3 system by CeO2 retards the thermal decomposition of PdO. This is opposite to results observed for Pd/CeO2 where the ceria accelerates the thermal decomposition of PdO relative to Pd/Al2 O3 . 2. NO TPD results show that Pd–Ce interactions affect the adsorption properties of palladium. The interaction was observed to be most significant between metallic palladium and surface reduced ceria species, and the preparation method played a role in the degree of interaction. More NO decomposition products were observed over the oxidized Pd/Al2 O3 catalysts than over the reduced sample, but this is most likely due to a decrease in the

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active surface of the pre-reduced catalyst caused by carbon deposition during reduction with CO. However, Pd oxide in the presence of CeO2 , is much less effective for NO decomposition. In return, an improvement is observed after reducing the PdCe/Al2 O3 catalysts, which is attributed to Pd–Ce interactions. This effect also favors N2 over N2 O formation. 3. With regard to the implications for three-way catalysts, the present results indicate that interactions between metallic Pd and ceria should improve Pd performance for NOx reduction. Future research should be directed in such a manner to find how Pd can be stabilized in the metallic phase, for example by using other supports (e.g. Ce-Y-Zr oxide, CeO2 –ZrO2 . . . etc.), in order to ensure ‘beneficial’ Pd–Ce interactions.

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